This invention relates to an anti-lock control system for vehicle wheel brakes in which the reapply pressure following an incipient wheel lockup condition is based on the brake pressure and wheel deceleration corresponding in time to the maximum tire torque tending to accelerate the wheel prior to the lockup condition and the brake pressure and wheel acceleration corresponding in time to the maximum tire torque tending to accelerate the wheel during wheel speed recovery following the lockup condition.
When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters including the road surface condition and the amount of slip between the wheel and the road surface. For a given road surface, the force between the wheel and the road surface increases with increasing slip values to a peak force occurring at a critical wheel slip value. As the value of wheel slip increases beyond the critical slip value, the force between the wheel and the road surface decreases. Stable braking results when the slip value is equal to or less than the critical slip value. However, when the slip value becomes greater than the critical slip value, braking becomes unstable resulting in sudden wheel lockup, reducing vehicle stopping distance and deterioration in the lateral stability of the vehicle.
U.S. application Ser. No. 789,576 filed on Oct. 21, 1985, now U.S. Pat. No. 4,664,453, and assigned to the assignee of this invention describes a wheel lock control system for preventing the wheels of a vehicle from locking up while being braked. In this system, the wheel brake pressure that results in the wheel slip being substantially at the critical slip value and which produces substantially the maximum braking force between the tire and the road surface is identified. When an incipient wheel lockup condition is detected, the brake pressure so identified is then applied to the wheel brake so as to substantially continuously establish the critical slip value between the wheel and the road surface resulting in the maximum possible braking effort.
The brake pressure producing the critical slip value and therefore the maximum braking force for a vehicle wheel is identified in the above system by repeatedly calculating the tire torque tending to accelerate the wheel during braking based on an equation defining the motion of a free body consisting of the wheel, tire and the brake. This equation utilizes measured values and system constants that are based on, for example, brake lining coefficient of friction and area and wheel radius. The measured brake pressure corresponding in time to the peak calculated tire torque is stored. When an incipient wheel lockup is detected indicating that the critical wheel slip value establishing the peak braking force between the wheel and road surface has been exceeded, the stored brake pressure is the pressure producing substantially the peak braking force. After detection of an incipient wheel lockup condition, the process of identifying the brake pressure producing the peak braking force is ended and the brake pressure is dumped to allow the wheel to recover from the incipient wheel lockup condition. When recovery is sensed, the stored brake pressure producing substantially the peak braking force is reestablished to establish a braking condition in which the wheel slip is substantially at the critical slip value for the existing road-tire interface condition.
In this system, at the time the peak tire torque is detected, the stored brake pressure value corresponding in time thereto may not be precisely the value producing the peak braking force due to wheel dynamics that comprise a portion of the calculated tire torque.